1. Field of the Invention
The present invention relates to a method of controlling tension in a continuous annealing furnace provided therein with tension control means, and a system therefor.
2. Description of the Prior Art
Recently, annealing processes for rendering predetermined processability, deep drawing properties and the like to cold-rolled steel strips have been carried out by continuous annealing furnaces. These continuous annealing furnaces each comprise a heating zone for heating the steel strip to a predetermined temperature, a soaking zone for holding the steel strip at a predetermined soaking temperature and a cooling zone for cooling the steel strip to substantially room temperature. The cooling zone further includes a rapid cooling zone for rapidly cooling the steel strip at a predetermined cooling rate, a slow cooling zone for slowly cooling the steel strip or holding same at a predetermined temperature to effect overaging treatment, and the like. Consequently, the above-described continuous annealing furnace generally forms a long continuous line, and therefore, it is necessary to render appropriate tension to the steel strip in the furnace in order to maintain stabilized operating conditions in the furnace.
FIG. 1 is an explanatory view showing a general example of the conventional continuous annealing furnace. As shown in FIG. 1, the continuous annealing furnace comprises a heating zone 1, a soaking zone 2, a first cooling zone 3, a second cooling zone 4, and a third cooling zone 5, bridle rolls 6a, 6b are provided in front and behind the furnace, and further, a tension control unit 7 is interposed between the bridle roll 6a and the heating zone 1. A steel strip 10 is loaded in order of the zones in the abovedescribed arrangement, and subjected to heat treatment. Namely, the steel strip is heated to a predetermined temperature in the heating zone 1, held at a predetermined temperature in the soaking zone 2, thereafter, passes through the first cooling zone 3, the second cooling zone 4 and the third cooling zone 5 while being successively cooled. The cooling rates in the respective cooling zones may be varied depending upon the compositions of the steel strip material to be treated and the intended characteristics of the material quality thereof.
Now, to control the steel strip tension in the furnace in this conventional example, tensions of the steel strip at the inlet and the outlet of the furnace are generally set. The actual adjustment of the tension is performed by means of a dancer roll provided between the bridle rolls disposed at the inlet of the furnace and the outlet of the furnace and with this arrangement the tension of the steel strip in the respective blocks in the furnace is not controllable. Consequently, proper tension has not been given to the steel strip in the respective cooling zones, thus presenting problems such as buckling in a non-aligned fashion, and slip, all of which are caused by unfitness and instability in tension of the steel strip. In order to obviate such problems, for example, in Japanese Patent Application Publication No. 30928/77, there has been disclosed such a method that the interior of a continuous heat treating furnace is divided into a plurality of blocks, and tension on the steel strip in the respective blocks are controlled in association with tension of the steel strip in the preceding and succeeding blocks. Namely in FIG. 1, tension meters 8a, 8b, 8c, 8d and 8e are provided in the furnace for detecting the tension of the respective sections of the steel strip. Output signals of tension meters represent the detected tensions in the respective sections, and are fed to steel strip tension control means 9a and 9g for controlling motors TM and M1 to M20. Each of the motors M1 to M19 drives each helper roll H1 to H19 for guiding the steel strip, individually from each other. The torque motor TM operates the tension control unit 7, and the motor M20 operates the bridle rolls 6b. The bridle rolls 6a are operated by a motor M21. More specifically, output signals from the tension meter 8a are fed to the steel strip tension control means 9a, 9b and 9c, outputs from the tension meter 8b to the steel strip tension control means 9b, 9c and 9d, outputs from the tension meter 8c to the steel strip tension control means 9c, 9d and 9e, outputs from the tension meter 8d to the steel strip tension control means 9d, 9e and 9f, and outputs from the tension meter 8e to the steel strip tension control means 9e, 9f and 9g. As described above, the respective tension meters feed their outputs to the groups of the steel strip tension control means of the block in question and the groups of the steel strip tension control means in the blocks preceding and succeeding the block in question. In addition, the tension command signals TS.sub.1 to TS.sub.5 are fed to the respective steel strip tension control means 9b to 9f for setting optimum tension in the respective sections of the steel strip. Furthermore, a tension setting signal TSC for setting the tension of the tension control unit 7 is fed to the steel strip tension control means 9a for driving the tension control unit 7.
In the arrangement of FIG. 1, deviation of tensions value between the detected tension value and the set tension value are obtained for each zone in the furnace, and the deviation tension values thus obtained are combined with detected tension values in the preceding and succeeding zones or a detected tension value in the preceding or succeeding zone to be used for controlling the torque of a motor or motors for a roll or rolls. With the arrangement as described above, it becomes possible that a preset distribution of tension in the furnace is maintained and the set tension values in the respective zones in the furnace can be automatically switched successively or simultaneously.
Nevertheless, the conventional control means present the following disadvantages.
(1) In the case a line speed, which is given as the master speed for the furnace, is based on the bridle roll 6b, the speed of the bridle roll 6b at the outlet of the furnace is varied depending upon the tension of the steel strip of the final cooling zone, whereby the speed of the bridle unit at the outlet is varied.
(2) Since a tension command signal for each zone is calculated from the detected tensions of the steel strip in the preceding and/or succeeding zones, the change of the tension command signal in a given zone affects the tension command signals in other zones, whereby the tension control is not stabilized. Also a fluctuation in deflection between the set value and the detected value of the tension affects the tension command signals in whole of the zones.
(3) Since the steel strip is given tension a high temperature in the heating and soaking zones, the steel strip is elongated due to plastic deformation depending upon the dimensions and temperature of the steel strip.
In this case, in principle, it suffice to hold the tension only in the heating zone and soaking zone at proper values. However, in the example shown in FIG. 1, the control of tension in the blocks preceding and succeeding the block in question, which are principally irrelevant to the block in question, are subject to the influence of the tension in the block in question, so that a stable tension cannot be obtained. Particularly, the influence is high in the case of the materials to be annealed at high temperature. For this reason, even in the case proposed as above, such problems have not been obviated as the movement in a non-aligned fashion, buckling, slip and the like of the steel strip, all of which are caused by unbalance in tension generated in the steel strip.